Method for purifying naphthenic acids



A rfill 2%, E9360 R. F. NELsN ET AL METHOD FOR PURIFYING NAPHTHENIC ACIDS Filed June 13, 1935 atented Apr. 28,

METHOD FOR PURIFYWG NAPHTHENIO ACIDS Roy F. Nelson, Nader-land, Tex., and lLouis Zapf, Dover, N. J., assignors to The Texas Eompany,

New York, N. Y., a corporation of Delaware Application time is, loss, Serial No. cases 6 illlaims.

This invention relates to a process for separating and recovering naphthenic acids from petroleum hydrocarbons, and especially to a process for recovering purified naphthenic acids from the residues obtained by vacuum-distilling naphthene base lubricating oil stocks with caustic soda or equivalent alkalies.

More specifically, our invention contemplates the segregation of the distillation residues obtained by vacuum-distilling naphthene base lubricating stocks in the presence of alkali metal hydroxides, and emulsifying the same with water in the presence of a low boiling hydrocarbon,

acidulating the so-formed emulsion with a mineral acid until the naphthenates, contained in the distillation residues, are decomposed and the free naphthenic acids liberated. The acidulated mix ture is then permitted to remain quiescent whereby there are formed two layers, the low comprising water, tar, salts, and free mineral acid, while the upper supernatant layer consists essentially of crude naphthenic acids dissolved in the diluent low boiling hydrocarbon. .The latter is then subiected to treatment as by distillation, whereby the .crude naphthenic acids are freed from the low boiling petroleum hydrocarbon diluent, whereupon the crude acids are subjected to a vacuum distillation to produce naphthenic acids of a high degree oi purity.

Naphthenic acids are cyclic organic acids which are present in varying amounts in a large number of crude petroleums of the naphthene base type. In the typical American crude oils, only minute percentages are present in the low viscosity derivatives, such as naphtha, kerosene and low boiling lubricating oils, the most of the acids being found in higher boiling fractions. In the refinery practice, it is customary to subject the topped crude to vacuum distillation in the presence of alkalies such as caustic soda, whereby the naphthenic. acids are converted into the corresponding non-vaporizable alkali salts. The residues remaining in the vacuum stills, after substantially all of the distillable fractions have been vaporized, comprise essentially the heavy hydro.- carbon oils, tars, and the aforementioned naph-, thenic acid salts. This mixture is spoken of in refinery terminology as lubricating still bottoms or lubricating still residues.

Various methods have been proposed for recovering naphthenic acids in a purified form from the lubricating still bottoms, with which'they are associated, inthe form of sodium salts. Among these, one of the most efilcient appears to be the extraction of the sodium naphthenates from the (i311. 26iL-WE) lubricating still bottoms by means of alcohol and thereafter acidulating the extracted sodium naphthcnates with sulfuric acid to liberate the corresponding naphthenic acids. However, this method is generally recognized to be uneconom- 5 ical because of the high cost of the extracting solvent. By the method which we have discovered, we are enabled to prepare naphthenic acids of a high degree of stability and purity without the disadvantage of employing expensive reagents or solvents. I

We have found that the naphthenic acids may be separated directly from the lubricating still bottoms by emulsifying the same with water, and then adding to the s'o-formed emulsion a low boiling diluent petroleum hydrocarbon which serves certain definite and important functions which will be fully discussed hereinafter. The mixture formed in this way is treated with a suflicient quantity of. sulfuric acid to decompose all of the sodium naphthenate present and to thereby liberate the corresponding naphthenic acids. When the acidulation step has been com pleted, the mixture is permitted to remain quiescent in order to permit settling of the water solution of sodium sulfate and free sulfuric acid and the precipitation of the tars which are associated with the sodium naphthenates in the lubricating still bottoms. The supernatant oily layer comprises a solution of the liberated naphtheme acids dissolved in a diluent low boiling hydrocarbon. This oily layer is then drawn ed and is subjected to distillation to remove the diluent, and the remaining naphthenic acids may then be further purified by means of a dispoints, all of which are characterised by their high degree of purity.

Our invention will be fully understood from the following description taken in conection with the drawing-which shows in diagrammatic sectional elevation an apparatus which may be used with advantage for carrying out the invention.

The numeral it designates a fractionating tower, into a lower point of which there is connected the line ill, controlled by the valve it. The top of the tower is connected through the line M, and the valve ill, with the coil it of the condenser Ill. The condenser coil discharges through the line it, controlled by the valve it, into the separator 20. The vapor space oi this separator is connected with a suitable evacuating :apparatus, such as a vacuum pump or iet. e

bottom 01' the separator is connected by line 2|, and valve 22, with pump 23, which discharges through line 24, and valve 25, into the storage tank 28. a v

The bottom of the fractionating tower I0 is connected through the line 21, valve 28, pump 29, and valve 30, with the agitator 3|, preferably provided with a conical bottom. It is also provided with a stirrer 32, which serves to agitate the contents. The numerals 33, 36, and 39 designate three storage tanks for the storage, respectively, of sulfuric acid, water and diluent hydrocarbon. These tanks are connected, respectively, to the agitator 3|, by the lines 34, 31, and 40, equipped with suitable control valves. The bottom of the agitator 3| is connected by means of the line 42, and valve 43, with the settling chamber 45.- This may, with advantage, be an I elongated vessel provided with a conical bottom,

the line 52, and valve 53, with the coil 5t of the ,89, provided with valve 90, connects with,

preheater 55. This coil discharges through the line 56, and expansion valve 51, into a lower part of the fractionating tower 58. This is provided at the bottom with the drawoil line 59, controlled by the valve. 60, and at the top with a cooling coil fiL-which serves to' supply reflux liquid during operation. The top of the fractionating tower is connected by line 53, and valve 64, with the coil 65, of the condenser 66. The condenser coil discharges through the line 51, and valve 68, into the receiving tank 69. This tank serves a dual purpose. It acts as a receiving tank for the diluent hydrocarbon oil, which has been distilled from the naphthenic acids, and serves also as a settling chamber wherein any water, which is distilled over with the distillate, is separated therefrom. The water drawofi line 10, leading from the bottom of the receiving tank 69, has a float-controlled valve H, which serves to maintain a small but constant level of water within the receiving tank. The top of the receiving tank is connected by the line 12, controlled by the valve 13, with the pump 1 2. This pump discharges through the line I5, and the valve I6, into the diluent hydrocarbon storage tank 39.

The drawofl line 59, at the bottom-of the tower 58, connects with the intake side of the pump I'l, which discharges through line I8, and valve I9, into the coil 80, of the preheater 8i. The

outlet side of the preheater coil is connected through the line 82, and expansionvalve 83, into a lower point of the fractionating tower 84. This tower is preferably so constructed as to permit fractionation therein at low pressures. The bottom of the tower is provided with a drawofi line 86, and valve 81, leading to suitable storage. tower, provides for the. formation of reflux liquid. From the top of the tower, the vapor line the coil 9|, of the condenser 92, which discharges through the line 93, controlled by the valve .94, into the receiving tank 95. The vapor space of this receiving tank is connected through line 96, and valves 91 and 93, with a vacuum pump or other evacuating means. The bottom of the receiving tank 95 is connected through the line The cooling coil 88, at the top of theconnected by means of the line I I2, andvalve I13, with the vacuum receiving tank H4. The upper portion of this receiving tank is connected through the line 5, and the valve 6, with the line 95, at a point intermediate the valves 91 and 98. The bottom of the vacuum receiving tank H4 is connected through line 8, pump I20, and line I2I, with an atmospheric storage tank I23.

In a typical operation of the process, in connection with the hereinbefore described apparatus, a lubricating oil stock is reacted at an elevated temperature with an alkali metal hydroxide such as, for example, sodium hydroxide, sufiicient to convert the naphthenic acids present in lubricating oil stock into sodium naphthenates. The heated mixture is discharged through the line II, into the fractionating tower III, which is maintained under a high vacuum, and the vaporizable fractions are taken off as distillates.

Although the accompanying figure shows provision for taking off only an overhead fraction, it is to be understood that one or more side stream fractions may be drawn ofi".

The non-voltatile residual fraction, which is formed, consisting of sodium naphthenates, tars and heavy relatively non-vaporizable oils, and heretofore spoken of as lubricating still bottoms or lubricating still residues, is drawn out of the 7 chamber 45, where the aqueous liquor, containing free sulfuric acid and sodium sulfate, to-

gether with tars and the like, settle out, leaving a supernatant layer of a solution of naphthenic acids in the diluent hydrocarbon oil which layer may be washed, if desired, with water. This solution is then delivered into the preheater 55, where it is heated to a temperature above the boiling point of the diluent hydrocarbon oil. The heated mixture is flashed into the fractionating tower 58, where substantially all of the diluent hydrocarhon oil is vaporized. The vapors are conducted to the condenser 66, where they are condensed and then pumped from the receiving tank 89, into the storage tank 39.

The non-vaporizable residue, which collects as a pool at the bottom of the tower 58,-is drawn off through the line 59, by means of the pump 11, and delivered by it under superatmospheric pressure into the preheater (H, where it is raised to an elevated temperature, preferably in the range of from 500 to 700 F. The heated mixture is then delivered into the fractionating tower 84, which is maintained under a high vacuum, where the naphthenic acids are substantially vaporized and pass upwardly through the tower. Cooling is supplied at an upper point in the tower in order to cause the formation of reflux liquid.

aoaaio The lower boiling naphthenic acids are drawn ofi as vapors from the top of the tower, while the higher-boiling naphthenic acids are drawn ofl at lower points, only one of which, however, is shown in the drawing. The naphthenic acid vapors, which are drawn ofi from the top of the tower, through the vapor line 89, are delivered into the condenser 92 where they are condensed. The condensate is delivered into the separatory chamber 95, the vapor space of which is connected with a vacuum pump or other means for evacuating the last distillation stage. The liquid space of the separatory chamber is connected by means of the line I00, with the pump W2, which serves to withdraw the collected condensate from the separatory chamber and deliver it into the storage tank I 05. The side stream of liquid naphthenic acids, which is drawn off through the line IN, is handled substantially in the same manner as the vapors of naphthenic acids taken ofi from the top of. the tower. They are cooled in the cooler Hi], from which they pass into the separatory chamber Mt. From this point they are pumped by means of the pump till, into the storage tank M3.

In carrying out the method of purification which forms the basis of this invention, the diluent hydrocarbon oils are used in the steps which involve the decomposition of sodium naphthenates and the subsequent distillation operation. It serves several functions, the most important of which is to make possible a rapid and efiective decomposition of the sodium naphthenates by the mineral acid employed. It also acts as a solvent for the naphthenic acids which are liberated and, in this manner, enables their separation from the aqueous liquor which is formed as a by-prodnot of the neutralization reaction. In the succeeding step, which involves the separation of the naphthenic acids from the diluent hydrocarbon, the diluent acts as a distillation carrier for some of the unstable hydrocarbons which are normally associated with impure naphthenic acids, making possible their vaporization and consequent separation during the distillation operation.

For this purpose, various types of petroleum hydrocarbons may be used, including kerosene,-

high boiling naphtha or low boiling naphtha such as precipitation naphtha or cut-back naphtha. Generally speaking, it isdesirable that the diluent hydrocarbon oil be comparatively low boiling so that it may be separated efiiectively and completely from the naphthenic acids by distillation.

For the most effective application of the present process, several important considerations must be recognized. The contaminating impurities in commercial naphthenic acids consist essentially of high boiling oils which apparently are unstable and gradually oxidize, thereby 1mparting color andother' undesirable features to the naphthenic acids. In order that the percentage of these undesirable oils may be kept down to a minimum, it is desirable that the distillation of the naphthene base oils, in the presence of caustic soda, be carried out under the highest vacuum that is economically possible, thereby insuring the removal of as much of the oils as possible.

Also, it is essential for the most efiective operation of the present process that provision be made for carrying out the final fractionation of the naphthenic acids under as ideal distillation conditions as is possible. This includes the provision of a carefully designed fractionating tower, preferably lagged with heat insulating material, so as to conserve as much of the heat as possible, and to prevent unduly high condensation within the fractionating vessel. Also it is desirable to use corrosion resistant materials for the iractionatlng and condensing equipment. Metals, which are useful for this purpose,'are aluminum, chromium and the various types of stainless steels which are now available on the market. It has been found experimentally that, by employing the aforementioned materials in the fractionating and even in the condensing and cooling apparatus, the products have a better color stability than normally obtained when iron or steel materials are employed in the construction of the distillation equipment. Although no theoretical explanation is oflered, it is thought that the naphthenic acids react with iron, thereby forming the dark brown iron naphthenate which, if carried over into the distillate stream, discolors the product and renders it more susceptible to oxidation.

Obviously many modifications and variations I carbon oils, which comprises distilling the oils in the presence of an alkali to remove distillate fractions thereof and form a residual oil fraction containing the neutralized naphthenic acids, emulsifying the residual fraction with water in the presence of a diluent light oil, acidifying the soiormed emulsion to liberate the naphthenic acids therein, subjecting the acidifled'mixture to settling to form a supernatant layer comprising a solution of the liberated naphthenic acids in the diluent oil, separating and removing the supernatant layer, separating therefrom the diluent oil, and then vacuum-distilling the remaining naphthenic acids to segregate naphthenic acids of the desired purity.

2. The method of recovering naphthenic acids of a high degree oi purity from the petroleum hydrocarbon oils in which they are dissolved, which comprises vacuum-distilling the oils in the presence of an alkali metal hydroxide to remove distillate fractions thereof and form a substan tiallynon-vaporizable residual fraction containing the alkali metal salts of the naphthenic acids, emulsifying the residual fraction with water in the presence of a diluent light oil, acidifying the so-i'ormed emulsion to decompose the alkali metal naphthenates and to liberate therefrom the naphthenic acids, then subjecting the acidified mixture to settling to form asupernatant layer com prising a solution oi the naphthenic acids in the diluent oil, separating the supernatant layer and subjecting it to distillation to remove the diluent oil and to form a residual fraction comprising naphthenic acids, and then subjecting the residual naphthenic acids to vacuum distillation tially non-vaporizable residual fraction containing the alkali metal salts of the naphthenic acids, emulsifying the residual fraction with water in the presence of a diluent light oil, acidiiying the so-iormed emulsion with a mineral acid to decompose the alkali metal naphthenates and to liberate the naphthenic acids, then subjecting the acidified mixture to settling to form a supernatant layer comprising a solution of the naphthenic acids in the diluent oil, separating the supernatant layer and subjecting it to distillation to remove the diluent and to form a residual fraction comprising naphthenic acids, and then subjecting the residual naphthenic acids to vacuum distillation to segregate the desired naphthenic acids.

4. The method of recovering substantially oilfree naphthenic acids from topped naphthene base crude oil in which they are dissolved, which comprises vacuum-distilling the crude oil in the presence of an alkali metal hydroxide to remove substantially acid-free distillate fractions thereof and form a substantially non-vaporizable residual fraction containing alkali metal salts of the naphthenic acids, emulsifying the residual traction with water in the presence of a diluent light oil, acidifying the so-formed emulsion with strong sulfuric acid to decompose the alkali metal naphthenates and to liberate the naphthenic acids, subjecting the acidified mixture to settling to form a'supernatant layer comprising a solution of the naphthenic acids in the diluent oil, sep-- arating the supernatant layer, subjecting it to degree of purity.

5. The method of recovering substantially oiliree naphthenlc acids from topped naphthene base crude oil in which they are dissolved, which comprises vacuum-distilling the crude oil in the presence of an alkali metal hydroxide to remove substantially acid-free distillate fractions thereof and form a substantially nonvaporizable residual fraction containing alkali metal salts of the naphthenic acids, emulsifying the residual fraction with water in the presence of naphtha, aciditying the so-iormed emulsion with strong suliuric acid to decompose the alkali metal naphthenates and to liberate thenaphthenic acids, subjecting the acidified mixture to settling to form a supernatant layer comprising a solution of the naphthenic acids in the naphtha, separating the supernatant layer, subjecting it to distillation to recover the naphtha and to form a residual traction comprising naphthenic acids, and then subjecting the latter to vacuum distillation to segregate naphthenic acids of the desired degree of purity.

6. The method of recovering naphthenic acids 01 a high degree of purity from petroleum hydro carbon oils, which comprises distilling the oils in the presence of an alkali to remove distillate fractions thereof and form a residual traction containing the alkali soaps of the naphthenic acids, decomposing the naphthenic acid soaps with a mineral acid in the presence of water and a diluent light oil, separating and removing the diluent oil solution of the naphthenic acids, removing therefrom the diluent, and then vacuum-distil ling the remaining naphthenic acids to segregate acids of the desired purity.

ROY F. NEISON. LOUIS ZAPF. 

